Clay Cracking During Drying
Here are some factors that contribute to drying problems:
The less all of these are true in your case the better drying experience you will have. Notice that 'fast drying' is not considered an issue, "uneven drying" is the problem. If you can fast-dry ware evenly you will have better success than slow-drying ware unevenly.
It is impossible to dry this clay. Yet I did it. How?
These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. It should be impossible to dry them, the most bentonite bodies can normally tolerate is 5%. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (but the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the better the joins, the more even the water content is throughout the piece during the entire drying cycle and the more damp of a climate you live in the better your drying success will be. What did it take to dry these: 1 month under cloth and plastic! I changed the cloth every couple of days. So by implementing these same principles you will have better drying success.
Wanna throw porcelain plates with thick bottoms and thin rims?
Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily, even though the bat had been dry. The base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.
Cross section is an important factor in avoiding cracks
The cross section of a bowl. For the best success in drying and firing, it is advantageous to have as even of a thickness as possible. But it is also important not to have sharp concave angles. It would have been possible to make the section outside the foot ring thinner by creating a more abrupt concave contour, but that contour, if too sharp, could offer a point of weakness where a crack could start.
One reason why stoneware clays are more convenient
These were all made at the same time and dried in the same way. Left: Three out of five have cracked on the bottoms. Right: None have. Guess which is the porcelain? Of course, I could have been more careful and dried the porcelain ones more slowly. That is the trade-off, more care (and expense) to get a whiter more vitreous result. So, for production, it makes sense to use a stoneware if at all possible.
An example of an S-crack in the bottom of a fired porcelain mug
Its shape, growth during the firing and penetration of glaze down into the crack demonstrates it preexisted firing (happened during the drying).
A crack on the bottom of a bisque fired mug
The stresses that created it were likely already present in the dried ware.
This horizontal crack began as stresses created during uneven drying
The rim was allowed to get ahead of the base. A thinner section (that happened during throwing) was exploited by the stresses and a crack appeared during heatup, likely during the bisque.
Stonewares dry better than porcelains
The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.
It shrinks much more yet cracks less. How is that possible?
Two mugs have dried. The clay on the left shrinks 7.5% on drying, the one on the right only 6%. Yet it consistently cracks less! Not the slightest hairline crack, not even at the handle joins. Why? Green or dry strength. If the dry clay matrix has the strength it can resist cracking even if there are stresses from uneven drying. The clay on the right is made using Kentucky ball clay, which has good plasticity but fairly low drying strength. The clay on the left is a native terra cotta, very plastic and very strong in the green state (likely double or triple the white clay). To demonstrate further: If paper fiber were added to the white clay, it would not crack. Why? Not because it would shrink less with the added fiber, no, the shrinkage would stay the same. Increased strength imparted by the fiber would give it the power to resist cracking.
Do grog additions always produce better drying performance?
This DFAC test for drying performance compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise there appears to be no improvement. While the grog addition reduces drying shrinkage by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Another issue is the jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.
This much grog was with that much clay!
This is a sculpture body named Industrial Crank from Potclays in the UK. I dried some out, slurried and screened the grog out then dewatered the remaining clay to get this. There is almost 50% grog. Yet this body is known for amazing plasticity and toughness. How is that possible with this much grog? That base clay. It is extremely sticky and plastic. Yet it has only 6% drying shrinkage. The grog has a narrow range of sizes, from about 35 mesh to 70 mesh (a high percentage is of the coarsest size). Yet amazingly, the body does not feel coarse. Why? Again, it is this clay base. Each grog particle is nestled in a buffer that firmly holds it yet gives it freedom to move. I am working on a complete report and will share from my Insight-live.com account soon.
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